Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art exhaust a complex mixture of air pollutants. These air pollutants are composed of gaseous compounds including, among other things, the oxides of nitrogen (NOX). Due to increased awareness of the environment, exhaust emission standards have become more stringent, and the amount of NOX emitted to the atmosphere by an engine may be regulated depending on the type of engine, size of engine, and/or class of engine.
In order to comply with the regulation of NOX, some engine manufacturers have implemented a strategy called selective catalytic reduction (SCR). SCR is an exhaust treatment process where a reductant, most commonly urea ((NH2)2CO) or a water/urea solution, is selectively injected into the exhaust gas stream of an engine and adsorbed onto a downstream substrate. The injected urea solution decomposes into ammonia (NH3), which reacts with NOX in the exhaust gas to form water (H2O) and diatomic nitrogen (N2).
In some applications, the substrate used for SCR purposes may need to be very large to help ensure it has enough surface area or effective volume to adsorb appropriate amounts of the ammonia required for sufficient reduction of NOX. These large substrates can be expensive and require significant amounts of space within the engine's exhaust system. In addition, the substrate must be placed far enough downstream of the injection location for the urea solution to have time to decompose into the ammonia gas and to evenly distribute within the exhaust flow for the efficient reduction of NOX. This spacing may further increase packaging difficulties of the exhaust system.
Exhaust backpressure caused by the use of the SCR substrate described above can be problematic in some situations. In particular, the SCR substrate can restrict exhaust flow to some extent and thereby cause an increase in the pressure of exhaust exiting an engine. If this exhaust back pressure is too high, the breathing ability and subsequent performance of the engine could be negatively impacted. Accordingly, measures should be taken to avoid overly restricting exhaust flow when implementing SCR.
The exhaust systems of many internal combustion engines can also be equipped with noise attenuation devices, such as mufflers. The mufflers are typically located downstream of the SCR substrates to dissipate excessive noise in the exhaust flow exiting the substrates. Although mufflers may help reduce some noise pollution, the inclusion of these serially-located devices often increases a size of the engine's exhaust system and, consequently, the difficulty of exhaust system packaging.
The exhaust system of the present disclosure addresses one or more of the needs set forth above.